The miniaturization of high intensity discharge (HID) lamps requires fabrication, placement and sealing of electrodes in tiny discharge vessels (also referred to as arc tubes or burners). Such HID arc tubes typically consist of transparent quartz or translucent polycrystalline alumina (PCA) bodies in which it is difficult to obtain hermetic seals, particularly at the smaller dimensions in low-wattage HID lamps. The high manufacturing costs of the electrode parts and high shrinkage due to manufacturing, placement, and handling issues further increase the difficulties encountered in mass producing low-wattage electroded HID lamps.
Electrodeless HID lamps offer an opportunity to have high speed, low cost, superior maintenance, precision lamps for low wattage applications since the associated problems with electrodes are eliminated. However, EHID lamps present a different set of problems which are primarily associated with coupling the energy from the high-frequency (HF) power supply into the arc tube. For example, air-filled helical resonators were designed to couple HF power to arc tubes in EHID lamps. Helical resonators produce axial electric fields in close proximity to the arc tubes to excite the discharge media within the arc tube. The size of the resonator depends inversely on the frequency. High frequency ISM bands around 915 and 2450 MHz were chosen to prevent electromagnetic interference (EMI) issues. However, as the lamps shrink in size, the field applicator becomes larger than the lamp causing optical shadowing effects.